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  • What is the pH of a solution of  NaOH solution?<div class='q

What is the pH of a solution of \mathrm{10^{-8}\ M} NaOH solution?

Option: 1

6


Option: 2

7.02


Option: 3

8


Option: 4

9.2


Answers (1)

best_answer

NaOH is a strong electrolyte and hence \mathrm{[OH^{-}]_{NaOH}= 10^{-8}\ M}

But pOH  \neq 8 because addition of a base will not cause the solution to become acidic.

The catch here is that the dissociation of Water also has to be considered.

\mathrm{H_2O(\ell) \rightleftharpoons H^+(aq) + OH^-(aq)}

    -                    \text{x}                \mathrm{\left (x+ 10^{-8} \right )}

Now, 

    \mathrm{[H^+] [OH^-] =10^{-14}}

\Rightarrow \mathrm{[x] [x+10^{-8}] =10^{-14}}

\Rightarrow \mathrm{x^2+10^{-8} x -10^{-14}=0}

On solving the quadratic equation

\Rightarrow \mathrm{x= \frac{-10^{-8}+\sqrt{10^{-16}+4 \times 10^{-14}}}{2}}

\Rightarrow \mathrm{x= 9.5 \times 10^{-8}= [H^+]}

\mathrm{Thus,\ pH = 7.02}

Hence, the correct answer is Option (2)

Posted by

Anam Khan

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